New York City municipal employees -- from firefighters to clerks -- are working under terms of union contracts that have expired, some as far back as 2009. This settlement backlog is expected to become the next mayor's problem.

And a problem it promises to be. According to the city's Independent Budget Office, a deal to hike wages by say, 2 percent across the workforce in each of two years could cost the city an immediate $3.8 billion from its $70 billion budget.

Democrats, the preferred party of organized labor, hold a 6-1 enrollment advantage over Republicans in the five boroughs. As limited as their wage promises will need to be, the party's candidates are pushing for labor endorsements one at a time, knowing that the unions are unlikely to line up behind a single primary contender.

"Nothing has changed all that much," said a longtime labor consultant who declined to be identified because of his clientele. "Unions are no more likely to all be doing the same thing than they were before. They'll be all over the map."

Besides, said an aide to one of the top-tier candidates, "This is not a matter of adding up endorsements and whoever gets the most wins. This is a race that the whole city will decide."

Candidates plumb their own connections for endorsements. Comptroller John Liu and ex-comptroller William C. Thompson Jr. have had union dealings in their official capacities. Council Speaker Christine Quinn is backed by the United Food and Commercial Workers Union, which among other things hailed her for opposing nonunion Wal-Mart's move to open in the city.

The UFCW's early nod has been the exception. One labor lobbyist said, under condition of anonymity, that he's been urging union officials: "Don't endorse too soon or you could get burned."

More than others in the crowded mayoral field, Public Advocate Bill de Blasio is closely identified with progressive labor causes -- a potential plus for him in a left-leaning primary. He won his current post in 2009 with key help from the Working Families Party. He's had longtime alliances with 1199/SEIU, the health care union and others. And de Blasio has been among the strongest advocates for the Paid Sick Time Act -- which would require businesses to allow employees some days off for illness if they do not do so already.

The bill has stalled in the council under Quinn -- a fact that de Blasio strategically targets. At his candidacy announcement Sunday, de Blasio said, "No New Yorker should have to choose between going to work when they're deeply sick and staying home and losing a day's pay that they can't afford to lose, or maybe even losing their job. That shouldn't be happening in 2013 and I will change that."

For unions, a candidate's chance of winning often factors into an endorsement decision. For candidates, the backing can mean a symbolic show of momentum -- as well as contributions and volunteers.